In the C3H/10T1/2CL8 cell line we have demonstrated that the expression of cells malignantly transformed by chemical carcinogens can be repressed by contact of malignant cells with non-transformed cells, and that this inhibition can be modulated by serum concentration and by inhibitors of phosphodiesterase. It is the major aim of this proposal to examine in greater detail correlations between alterations in cyclic nucleotide levels and alterations in growth inhibition, and to examine how this intracellular communication occurs in vitro. This will be achieved by investigating the growth inhibitory effects of drugs which alter cyclic nucleotide metabolism on levels of cAMP and cGMP determined by radioimmunoassay, and by the use of electron microscopy and radioautography to demonstrate the presence and functionality of gap junctional communication between cells in culture. The state of growth arrest induced in co-cultivated malignant cells will be studied by flow-cytofluorimetry and selective cytotoxicity using cell cycle phase specific cytotoxic drugs. The relevance of the observed in vitro effects to the in vivo situation will be investigated by studying the growth rate of transplantable tumors, of differing sensitivity to growth inhibition in vitro, in host animals treated with drugs which modify the in vitro response. We will also extend these studies to human fibroblasts in culture which we have shown to secrete an inhibitory factor into the medium. We will characterize this factor biologically and biochemically. These studies, in addition to providing basic information on growth control mechanisms, will aid in the development of more reliable and quantitative in vitro assays for chemically induced malignant transformation in mouse and human cell lines.